Why Frequency Coordination is Important!

20 Aug Why Frequency Coordination is Important!

Wireless frequency coordination is an under-valued skill among Wedding DJ’s, but it’s such an important aspect of the “tech” side of this business. Nothing wireless is 100% foolproof, but there are some best practices we can follow to make sure your wedding officiant doesn’t have to ask “Can you hear me now?”

A basic tenet of wireless transmission and reception is to make sure that you’re on a “clear” frequency — simply put: one that nobody else is operating on. But in reality, it’s actually much more complicated than that–particularly if you want to operate at long distance. On Earth – among all the technology we have down here – there’s really no such thing as a completely “clear” frequency, so much as a “frequency that’s got little enough activity that a microphone transmitter at a given distance will adequately stand out.”

So wireless reception is really the receivers ability to distinguish the carrier signal (your microphone transmitter) from everything else – also called the “carrier to noise ratio.” The other wireless transmissions that your signal compete with are therefore called your “noise floor.” It’s basically a wireless version of Where’s Waldo: on a completely blank page, it would be super easy to distinguish Waldo from white paper. In a crowded environment, it’s much harder. Wireless works the same way, and all the other people on the page are your “noise floor” when you’re looking for Waldo.

We operate in the UHF (TV) band which is unlicensed spectrum in the United States. So theoretically anyone at any time can fire up a UHF device (not to mention actual TV stations) and cause you interference. This is what my “noise floor” looked like when I showed up to a gig at Pippin Hill outside Charlottesville, VA this past weekend:

This was a simple snapshot in time. Along the bottom you can see TV channel numbers that correspond their given frequency allocations. The red line corresponds to the level which I’ve told the software = a no go zone and the blue vertical lines note those specific frequencies.

However, I always leave my scan going throughout load in and set up to make sure that I pick up as much noise as possible from any sources: kitchen equipment, other vendors equipment, LED lighting, wedding planner or venue walkie talkie systems, etc: hopefully anything that I might encounter during the actual usage of my equipment. This is what the noise floor scan over time looked like:

Every one of those blue lines represents a signal – caused by something external to my own equipment – that would be loud background noise for my wireless receivers if I tried to operate there… shortening my wireless range and potentially even causing RF “hits” or dropouts. How many of those signals were going to be present during the 30 minute wedding ceremony? Who knows? All I know is they don’t belong to me, so I can’t control them and it’s best to plan around them.

This is a fatal flaw of the “scan and sync” feature that’s so popular among most wireless systems. Many manufacturers and retailers even tout how easy this process is. It *is* easy, and for that reason most DJ’s do that and say “done” when setting up their wireless, but all they really know is that they’re on channel their receiver chose for them. They don’t know where the receivers own “red line” is, and they only know that they’ve scanned a quick snapshot in time.

Critically: the scan and sync feature also has no idea what TV stations are licensed in your area (unless they’re currently broadcasting in excess of that threshold–the one you can’t even quantify), so if a TV station only fires up during prime time, for instance, your scan & sync might miss it if you scan at 3pm. Here’s what the “intelligent” noise floor looks like including TV channels within reception range:

Compare what you see above to the first image — the snapshot in time scan [the “show up, scan and sync” version].

Relying on a single simple scan can REALLY get you into trouble. That “scan” of my location would’ve told me that operating anywhere between 536-548MHz was “clear,” when in reality, those TV stations (25 and 26) might increase their transmission power or even start broadcasting during prime time … which just so happens to coordinate nicely with wedding timelines. It seems like Channel 22 DID significantly increase their transmission power at some point while I was on site. Licensed TV stations broadcast in kilowatts (thousands of watts) and up to 1 megawatt (1 millon watts) of power while wireless mic users are limited to 50 milliwats (50 millionTHS of a watt) … so you can guess what happens if a licensed TV station fires up on “your” frequency during a wedding.

Being armed with all this information also allows me to play with that red line threshold. If I have a super low noise floor, I can be REALLY picky about which frequencies I want to use for maximum transmission range. If I have a crowded noise floor (this is Charlottesville, VA … imagine what NYC looks like), I have to raise that line to get less picky.

In any case, knowledge is power. And now YOU KNOW not to “scan and sync” (or hire a DJ that does).